Question

In c++, Is it possible to write to some kind of buffer with printf (or similar) and then later in the program either write the buffer to the screen or discard it depending on the outcome.

I want to do this because I have a recursive function and only want the see the things printed throughout the recursion if the result is of interest.

Answer 1

Certainly. You can leverage the power of vsnprintf for that purpose. I’d suggest some sort of class wrapping an std::string or std::vector<char> (essentially the same in C++11):

#include <cstdargs>
#include <cstdio>
#include <string>

class Formatter
{
    std::string buf;
public:
    void add(char const * fmt, ...)
    {
        std::va_list ap, aq;
        va_start(ap, fmt);
        va_copy(aq, ap);

        int ret1 = std::vsnprintf(NULL, 0, fmt, ap);
        // check ret1 != -1

        std::size_t cur = buf.size();
        buf.resize(cur + ret1 + 1);

        int ret2 = std::vsnprintf(&buf[cur], ret1 + 1, fmt, aq);
        // check ret2 != -1
        buf.resize(cur + ret1);

        va_end(aq);
        va_end(ap);
    }

    std::string const & str() const { return buf; }
};

Now you can say:

Formatter f;
f.add("Hello, %s", "world");
f.add("%i%i%i", 1, 2, 3);
std::cout << f.str() << std::endl;

If you’re very concerned about performance, you can try and preallocate some space for the print operation and maintain a separate “end” position, in the hope that you’ll never have to run the vnsprintf call more than once.